1. Field of the Invention
The present invention relates to a toner and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, an image forming apparatus employing an electrophotographic process, an electrostatic printing process, or the like process, has been widely applied to a copier, a printer, a facsimile machine, and the like machine. For example, an electrophotographic image forming apparatus forms an image onto a recording sheet as follows. By using a photoreceptor on which surface a photosensitive layer containing a photoconductive substance is formed for serving as an image carrier, an electrostatic latent image based on image information is formed on the photoreceptor through various image-forming processes. The electrostatic latent image is then developed to a visible image by use of developer containing a toner, which is supplied from a developer tank. And the visible image is transferred to a recording material such as paper, and is then heated and pressed against the recording material by a developing roller so that the visible image is fixed on the recording material.
In the image forming apparatus as described above, the toner is used for visualizing the electrostatic latent image formed on the image carrier. And a dry developing method using a toner generally includes a developing method which employs frictional electrification, such as a powder craft method, a cascade method, and a magnetic brush method. Among these methods, the magnetic brush method has been widely used because, in the method, the developing operation can be easily controlled and it is possible to obtain a high-quality recording image. The magnetic brush method is classified into two types: a one-component developing method in which only a toner containing a magnetic substance constitutes a magnetic brush for development; and a two-component developing method in which two-component developer containing a toner and magnetic particles called carrier constitutes a magnetic brush for development. In both of the methods, the development is performed in a manner that predetermined charges are imparted to the toner on the magnetic brush, and the toner is brought to an electrostatic latent image on a photoreceptor by the Coulomb's force. In the toner, colorant is dispersed in binder resin. The binder resin includes various types of synthetic resin which exhibit electricity-detecting characteristic and binding characteristic to a moderate degree, represented by styrene-based resin and polyester-based resin, for example. Examples of the colorant include carbon black, organic colorant, and inorganic colorant.
In the developing method employing frictional electrification, in order to form images of substantially constant quality without deterioration stably and for a long period of time, it is desired that the toner be charged up to an appropriate level immediately after the developer tank is replenished therewith and furthermore that in outputting images one after another for a long period of time, the toner do not have charges excessively accumulated or unnecessarily released even through continuing frictional agitation so that the charge amount fluctuates in a narrow range. Moreover, the toner is required to have charges thereof not fluctuating in various environments, particularly, even in a high humidity environment so that the charge amount is stable for a long period of time.
With the aim of obtaining a stable toner which exhibits a small change in charge amount in course of time, to the toner is commonly added a charge control agent such as a metal-containing azo dye, salicylic-acid metal complex, and quanternary ammonium salt. These charge control agents, however, do not have sufficient dispersibility into the binder resin, thus causing the charge amount of toner to remarkably fluctuate, with the result that an image thus obtained may be varied in quality.
In view of the conventional technical problems, diverse proposals have been made for enhancing the dispersibility of the charge control agent into the binder resin, the stability of charge amount of toner, and the like characteristics.
For example, Japanese Unexamined Patent Publication JP-A 11-338192 (1999) discloses a toner for electrostatic charge image which has specified structure and characteristics and contains a charge control agent having an excellent charging property. The above-stated charge control agent is excellent in durability and environmental stability and further excellent in transparency by virtue of its characteristics of having an acicular crystal and a faintly yellow color, thus being suitable for a color toner.
Further, Japanese Examined Patent Publication JP-B2 7-104620 (1995) discloses a negative electric charge exchange control agent which contains an organic boron compound composed of: an organic anionic component containing boron as a charge exchange control agent; and a cationic component having a molecular size smaller than that of the organic anionic component. However, in the above-stated JP-B2 7-104620, no shape or dispersed state of the charge exchange control agent is mentioned and moreover, sufficient environmental stability and temporal stability are not obtained.
Further, there is a known electrophotographic toner of conventional design in which a content of charge control dye per 1 g of toner, that is to say, a surface CCA concentration of charge control agent is 2.0×10−3 g/g to 9.0×10−3 g/g where the charge control agent is present on the surface of the toner. The electrophotographic toner of these types, however, involves a problem that a carrier is contaminated by a charge control agent which is dropped off from toner particles upon repeated use for a long period of time. Given the problem, Japanese Unexamined Patent Publication JP-A 5-27483 (1993) discloses an electrophotographic toner of which surface CCA concentration mentioned above falls in a range of 1.0×10−3 g/g to 1.7×10−3 g/g for solving the above problem. The electrophotographic toner of which surface CCA concentration falls in the above range is able to decrease the contamination of toner thanks to a smaller surface CCA concentration and thus a smaller amount of charge control agent dropped off from the surfaces of toner particles as compared to those of conventional design. However, JP-A 5-27483 contains no description about a particle distribution of toner. A large amount of small-diameter toner leads an increase in the amount of charge control agent which is exposed on the surface, and such a charge control agent will drop off upon a long-term printing occasion, thereby causing the charge amount to fluctuate and thus resulting in poor temporal stability of charges. Moreover, the environmental stability of charges also deteriorates since the charge control agent is high in ion conductivity and thus susceptible to moisture outside.